1. Field of the Invention
The present invention relates to a a group III nitride compound semiconductor device and a producing method therefor. Particularly, it relates to improvement in a buffer layer-forming method used when a group III nitride compound semiconductor device is produced by a metal organic chemical vapor deposition method (MOCVD method).
The present application is based on Japanese Patent Applications No. Hei. 11-222882 and 11-315193, which are incorporated herein by reference.
2. Description of the Related Art
Heretofore known is a group III nitride compound semiconductor device having a structure in which group III nitride compound semiconductors are laminated on a sapphire substrate through a buffer layer of AlxGa1xe2x88x92xN (0xe2x89xa6Xxe2x89xa61).
For example, Unexamined Japanese Patent Publication No. Sho. 62-119196 has described a buffer layer-forming method in which a buffer layer of AlxGa1xe2x88x92xN (0xe2x89xa6Xxe2x89xa61) is grown at a growth temperature of from 950 to 1150xc2x0 C. on a sapphire substrate heated to about 1000xc2x0 C. by the MOCVD method.
On the other hand, by the investigation thereafter, it has been found that the crystallinity of a GaN compound semiconductor layer grown on a buffer layer is improved when the buffer layer is grown at a lower temperature of about 400xc2x0 C. on a sapphire substrate heated to about 1000xc2x0 C. See Unexamined Japanese Patent Publication No. Hei. 2-229476, etc. Any main group III nitride compound semiconductor light-emitting device (LED, etc.) currently put into practical use uses such a low-temperature grown buffer layer.
Incidentally, the growth temperature means the temperature of a substrate heated in execution of an MOCVD method.
The film-forming temperature of a semiconductor layer formed on a buffer layer is, however, generally about 1000xc2x0 C. Hence, when the aforementioned low-temperature grown buffer layer is used, it is necessary that the substrate heated to about 1000xc2x0 C. is once cooled to about 400xc2x0 C. for the sake of surface cleaning and then heated to about 1000xc2x0 C. again. When the substrate temperature condition is changed in a sequence of high temperature low temperature high temperature in the aforementioned manner, it requires much time and labor to adjust the substrate temperature itself. Hence, the large change of the substrate temperature condition was a barrier against the improvement of efficiency in production of a semiconductor device in which a GaN compound semiconductor layer is grown through such a low-temperature grown buffer layer.
In order to solve the problem, a technique to form a buffer layer at a high temperature has been disclosed in Unexamined Japanese Patent Publications Nos. Hei.9-148626, 7-321374, 9-64477, and Sho. 59-57997, etc.
Further, with respect to nitriding of a surface of a sapphire substrate, see Unexamined Japanese Patent Publication No. Hei. 5-41541.
By the way, a general device structure of a group III nitride compound semiconductor device used for a light-emitting diode, or the like, is formed as follows. A thin buffer layer of AlN or GaN is formed on a sapphire substrate at a low temperature, and a group III nitride compound semiconductor layer, such as a GaN layer, constituting a device function is laminated on the buffer layer. In such a device, the buffer layer is guessed to be amorphous or polycrystalline. It is conceived that the buffer layer serves as a one-directionally oriented seed crystal at a high growth temperature (about 1000xc2x0 C.) for the formation of a device function layer and relaxes thermal distortion based on the difference in thermal expansivity between the device function layer and the sapphire substrate.
In opposition to such an amorphous or polycrystalline buffer layer, Unexamined Japanese Patent Publication No. Hei. 9-64477 has made a proposal that a single crystalline AlN buffer layer is formed on a sapphire substrate.
Unexamined Japanese Patent Publication No. Hei. 9-64477 has asserted that a single crystalline AlN layer good in crystallinity can be grown on the sapphire substrate when a condition is satisfied so that the buffer layer is grown up to a thickness of from 20 to 300 nm at a high temperature not lower than 1300xc2x0 C. and that the half-value width of an X-ray rocking curve of the buffer layer is set to be not longer than 90 sec.
AlN as a single crystal with good crystallinity can be surely grown on the sapphire substrate by the background-art method described in Unexamined Japanese Patent Publication No. Hei. 9-64477. If the crystallinity of the buffer layer is good, a device function layer of group III nitride compound semiconductors can be grown with good crystallinity on the buffer layer.
According to the present inventors"" examination, however, the crystallinity of the buffer layer formed by the method described in Unexamined Japanese Patent Publication No. Hei. 9-64477 is insufficient to form such a device.
An object of the present invention is to provide an AlN single crystal layer excellent as a buffer layer for a device function layer of group III nitride compound semiconductors to thereby solve the aforementioned problem.
Another object of the present invention is to form a single crystal AlN layer good in crystallinity on a substrate directly. Such a single crystal AlN layer has both high electrical insulation characteristic and high thermal conduction characteristic. At the same time, the single crystal AlN layer can be used for forming various types of semiconductor functional devices such as a high-frequency conversion device which is an acoustic device utilizing piezoelectric characteristic.
Still another object of the present invention is to provide a preferred condition for formation of a group III nitride compound semiconductor layer on a high-temperature buffer layer as proposed in the background art.
The present inventors have made investigation to achieve the foregoing object. As a result, it has been found that a group III nitride compound semiconductor layer having a crystallinity equal to or better than that of the background-art layer can be grown directly on a sapphire substrate even at an ordinary temperature, that is, at a high temperature for growing a group III nitride compound semiconductor layer when the thickness of the layer is set to be a predetermined value under a predetermined condition.
The present invention is devised on the basis of such knowledge. That is, there is provided a method for producing a group III nitride compound semiconductor device, comprising the step of growing, at a growth temperature of from 1000 to 1180xc2x0 C., a layer of a group III nitride compound semiconductor having a thickness of from 0.01 to 3.2 xcexcm on a substrate having a surface nitride layer having a thickness of not larger than 300 xc3x85.
In the producing method according to the present invention, a series of production steps of from the step of heating a substrate to the step of forming a group III nitride compound semiconductor layer can be performed without any large temperature change. As a result, the time and labor required for adjusting the substrate temperature in the background art can be reduced, so that the efficiency in production of the semiconductor device can be improved.
Further, according to the inventors"" observation, a second group III nitride compound semiconductor layer grown on a first group III nitride compound semiconductor layer formed in the aforementioned condition is excellent in morphology. Hence, the first group III nitride compound semiconductor layer is excellent as a buffer layer interposed between the second group III nitride compound semiconductor layer constituting a device function portion and the substrate.
Still further, the present invention is devised to achieve at least one of the aforementioned objects. According to the present invention, there is provided a group III nitride compound semiconductor device comprising a substrate, and an AlN single crystal layer formed on said substrate, the AlN single crystal layer having a thickness of from 0.5 to 3 xcexcm and having a substantially flat surface, wherein the half-value width of an X-ray rocking curve of the AlN single crystal layer is not longer than 50 sec.
According to the group III nitride compound semiconductor device configured as described above, the crystallinity of the AlN single crystal layer formed on the substrate becomes very good. Hence, a device function can be built in the AlN single crystal layer itself. Moreover, also in the case where a device function layer of group III nitride compound semiconductors is formed on the AlN single crystal layer, the crystallinity of the device function layer becomes equal to or higher than the crystallinity of a device function layer formed on a low-temperature growth buffer layer used generally.
In the above description, the substrate is not specifically limited so long as an AlN single crystal can be grown on the substrate. Examples of the substrate material may include sapphire, materials with spinel structure, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, group III nitride compound semiconductor single crystal, etc.
According to the inventors"" examination, it is preferable that sapphire, especially the face a thereof, is used as the substrate material.
The thickness of the AlN single crystal layer is set to be in a range of from 0.5 to 3 xcexcm. If the thickness is smaller than 0.5 xcexcm, it is insufficient to form a device function. Contrariwise, the thickness need not be set to be larger than 3 xcexcm.
This is because each of layers necessary for forming a semiconductor device is generally not thicker than 3 xcexcm though the layers may be thicker than 3 xcexcm in total. The reason why the AlN single crystal layer is thicker than that described in Unexamined Japanese Patent Publication No. Hei. 9-64477 is that the crystallinity of AlN itself appears. If the layer is too thin, the crystal is warped because of the influence of the substrate. Contrariwise, when the layer is thickened, the crystal""s own nature can be utilized.
The half-value width of an X-ray rocking curve as an indicator for the crystallinity of the AlN single crystal is set to be not longer than 50 sec. If the half-value width is longer than 50 sec, crystallinity sufficiently adapted to a semiconductor device cannot be secured. Also when a group III nitride compound semiconductor layer is grown on the AlN single crystal layer, the half-value width of the AlN single crystal layer is preferably set to be not longer than 50 sec from the point of view of flattening a surface of the AlN single crystal layer substantially to secure the stable crystal growth of the group III nitride compound semiconductor layer.
Group III nitride compound semiconductors formed on the AlN single crystal layer are represented by the general formula AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61, 0xe2x89xa6X+Yxe2x89xa61), which includes so-called binary compounds such as AlN, GaN and InN, and so-called ternary compounds such as AlxGa1xe2x88x92xN, AlxIn1xe2x88x92xN and GaxIn1xe2x88x92xN (0xe2x89xa6Xxe2x89xa61 each). The group III elements may be partially replaced by boron (B), thallium (Tl), etc. The nitrogen (N) may be partially replaced by phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), etc.
The group III nitride compound semiconductors may contain any optional dopant. Si, Ge, Se, Te, C, etc. may be used as n-type impurities. Mg, Zn, Be, Ca, Sr, Ba, etc. may be used as p-type impurities. After doped with p-type impurities, the group III nitride compound semiconductor may be subjected to electron beam irradiation, plasma irradiation or heating by a furnace.
The method of forming the group III nitride compound semiconductor layer is not specifically limited but the layer may be formed by a metal organic chemical vapor deposition method (MOCVD method) or by a known method such as a molecular beam epitaxy method (MBE method), a halide vapor phase epitaxy method (HVPE method), a sputtering method, an ion-plating method, an electron shower method, or the like.
Examples of the device constituted by the group III nitride compound semiconductor layer include optical devices such as a light-emitting diode, a photodetector, a laser diode, a solar cell, etc., bipolar devices such as a rectifier, a thyristor, a transistor, etc., unipolar devices such as an FET, etc., and electronic devices such as a microwave device, etc. The present invention may be applied also to laminates as intermediates of these devices. Incidentally, a homostructure, a heterostructure or a double heterostructure having an MIS junction, a pin junction or a p-n junction may be used as the configuration of the light-emitting device. A quantum well structure (single quantum well structure or multiple quantum well structure) may be used as the configuration of the light-emitting layer.
Any of the aforementioned devices may be built in the AlN single crystal layer.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.